What can go into a septic tank?

Septic Tank Dos and Don'ts

If you’ve got a septic tank, or you’re in the process of buying a septic tank for your property, one question you may be asking is “what can go into a septic tank?” 

If so, here’s a helpful overview of what can, cannot and should not find its way into your septic tank if you want to make sure it’s working as best as possible, for as long as possible. 

A septic tank is used to store, collect and process all wastewater from a domestic property. This is the black water, which is used water from toilets and dishwashers, containing both food waste and human waste. As well as grey water, which is the water that has been used in your showers, sinks, baths and basins. 

It’s not the place to send your surface water or rainwater from your guttering and downpipes to – this is a no no. Rainwater, stormwater, surface water, whatever you want to call it, must be separate to the water that drains to your septic tank. This was must go to either the mains surface water drainage network, if there is one nearby that you’re allowed to connect into, or it must go to a completely separate soakaway or stormwater attenuation tank. We would of course encourage you to consider using a rainwater harvesting system for this water too! All the rainwater you’re dealing with that falls on the roof of your property could be saved and re-used if you include a rainwater harvesting system for your home, which is an absolute no-brainer if you ever water your garden, irrigate your lawn, wash your car or flush your toilets! You’ll be used to using mains water for this and yet could be using harvested, recycled rainwater for all these applications instead. Like I said “no-brainer”.

Anyway, back to septic tanks… the mixture of black water and grey water that will make up the wastewater that enters the septic tank will mean all waste is collected into the primary chamber of the septic tank and the solids in that wastewater will sink to the bottom. During this process the cleaner water will naturally rise to the top. 

As the cleaner water rises to the top, it will flow into the second chamber where all the cleaner water is stored before overflowing out of the outlet pipe connection and into the drainage network which has to be either a soakaway or a drainage field, which is a network of perforated pipes usually buried underground in a continuous loop to ensure controlled effluent distribution throughout the drainage field. 

Here are some frequently asked questions that we’ve received, which may also be helpful for you to know the answers to…

Q: Will a septic tank flood itself?

A: It should not. If all is working correctly then it should not be possible for any part of the septic tank to flood. Only if there is a problem and perhaps a blockage in the outlet pipe or a blockage or damage to the drainage field or soakaway might the wastewater start to back-up into the incoming pipe. If this does occur then you’re likely to know about it soon enough because you’ll be able to smell the sewage inside the tank. It will build up and start to back-up the incoming pipe and the natural process of how the water is treated will be interrupted, so you’ll smell it and then address the issue. 

Q: Can a septic tank leak?

A: To answer the question very literally, yes, all underground tanks can leak. But they shouldn’t. There is something wrong if it does leak. Any material, whether concrete, plastic, GRP, steel or otherwise can of course suffer from damage and can leak. However, when the tank is installed it has to be filled with water during the backfilling process, so it would be noticed at this point if there was a leak in the tank. This would be the time to spot it and sort it, because once it’s buried and in the ground it’s a lot more problematic to change or fix the tank. There is no reason why the tank would leak if its been installed correctly and was in good condition when fitted. 

Q: Can a septic tank be uphill from a house?

A: Only if you have that magic kind of water that flows up hill. Sorry for our sarcasm, but no, a septic tank cannot be uphill from a house unless you collect all your wastewater into a pump chamber and then pump it up to the septic tank. What’s more common is to install the septic tank down hill from the property so it can drain into the tank via gravity and then if you have to pump it away to a higher level, you have a pump outlet after the septic tank which pumps the clean water to a higher level, rather than use a foul water pump to pump the water pre-treatment. 

Q: Can you fill a septic tank with rainwater?

A: As mentioned above, no, this is not what a septic tank is for. Buy a rainwater harvesting system for this and make the most of using harvested, free, rainwater for all your non-potable water requirements around the house, such as garden irrigation, washing your car, flushing your toilets and even your washing machine water supply. Just make sure you don’t send your rainwater to your septic tank.

Q: What can you put in a septic tank?

A: Wastewater. Sewage. Black & Grey water. That’s it. Keep it simple. There are also items that you won’t want to flush down your toilet to make sure you avoid blocking up your pipework to the septic tank as well. For a full list of these items, read our “Bin it, Don’t flush it” document, which can be found here.